The present invention relates generally to electrostatic spray guns, and more specifically to an improvement for dissipating electrical energy which becomes capacitively stored in the conductive components of electrostatic spray guns. The invention is an improvement in the design of such spray guns wherein a fluid spray valve serves the dual function of controlling fluid flow through a fluid ejection orifice at the forward end of a spray gun as well as carrying the electrically conductive path which terminates in an electrode projecting through the fluid ejection orifice.
Fluid spray valves incorporating an electrostatic electrode are known in the art. An example of an electrostatic spray gun having such a construction is shown in U.S. Pat. No. 3,583,632, issued June 8, 1971, wherein an axially movable spray valve carries an electrostatic electrode projecting through a fluid ejection orifice. The spray gun disclosed in this prior art patent has an electrical resistor incorporated into the body of the spray gun for damping capacitively stored electrical energy, but such resistor is located at a significant distance from the forwardly projecting electrode. The electrically conductive components between the resistor of the spray gun and the electrode are capable of storing a significant amount of capacitive energy, and if such energy is dissipated in the form of a spark in an explosive atmosphere, it can cause fire or explosion to result.
The placement of a smaller resistor at a position which is physically closer to the electrode is shown in U.S. Pat. No. 4,241,880, issued Dec. 30, 1980, wherein the smaller resistor is contained within the fluid spray valve, and is electrically connected to the projecting electrode. A resistor in this position dampens the capacitively stored energy which may be stored in the conductive components upstream of the resistor. The forward resistor is embedded in the fluid flow valve itself, and therefore is axially movable with the fluid flow valve as it is engaged by a trigger to permit the ejection of fluids from the fluid ejection orifice.
The same concept is taught in U.S. Pat. No. 3,233,831, issued Feb. 8, 1966 wherein the fluid spray valve itself may be made of resistive material, and also serves as the electrode, to provide the necessary resistive damping of capacitively stored energy to the forwardmost electrical point of the spray gun. Similarly, this patent discloses an axially slidable fluid spray valve, wherein the resistive component moves in coincidence with the spray valve.
The small, forwardmost resistors disclosed in the foregoing patents either form a part of the fluid spray valve or are contained therein, and thus necessarily must be constructed of small physical size to meet the design requirements of the spray valve. In the case of the '880 patent, the resistor must be constructed of very small phyical size so as to be capable of being embedded directly within the spray valve. The heat generated by current flow through this very small resistor is dissipated only by passing through the outer spray valve casing, thereby creating heat and electrical stress within the resistor. Further, since the spray valve itself is a component which is subject to considerable wear during the course of the spray gun operation, it is frequently manufactured as a replaceable component. Because the resistor is physically enclosed within the spray valve, replacement of the spray valve due to mechanical wear in use necessitates replacement of the resistor as well. It is therefore desirable to provide the requisite resistive component proximate the forward end of the spray gun, without also requiring that it be discarded as a part of the fluid spray valve whenever physical wear of the spray valve necessitates replacement. Further, it is desirable to construct the forwardmost resistor physically as large as possible in order to improve heat dissipation.